In the manufacture of tissue products such as bath tissue, a wide variety of product characteristics must be given attention in order to provide a final product with the appropriate blend of attributes suitable for the product""s intended purposes. Improving the softness of a tissue product has always been a major objective for premium products. The major components of softness include stiffness and bulk (density), with lower stiffness and higher bulk (lower density) generally improving perceived softness.
One traditional approach to producing tissue products has involved compression of a wet laid web between an absorbent felt and the surface of a rotating heated cylinder such as a Yankee dryer. The dried web is thereafter dislodged from the Yankee dryer in a creping process in which a blade is used to partially de-bond the dried web by breaking many of the bonds previously formed during the wet pressing stages of the process. Creping generally improves the softness of the web, albeit at the expense of a significant loss in strength.
More recently, throughdrying has become a more prevalent means of drying tissue webs. Throughdrying provides a relatively non-compressive method of removing water from the web by passing hot, dry air through the web until it is dry. More specifically, a wet-laid web is transferred from a forming fabric to a coarse, highly permeable throughdrying fabric and retained on the throughdrying fabric through passage of a xe2x80x9cthrough air dryerxe2x80x9d, (hereinafter TAD). The resulting dried web is softer and bulkier than a wet-pressed uncreped dried sheet because fewer paper-making bonds are formed and because the web is less dense.
Although throughdried tissue products exhibit good bulk and softness properties, throughdrying tissue machines are expensive to build and operate. Accordingly, there is a need for improvements for a throughdrying apparatus and process which produces high quality tissue products.
It has now been discovered that in the manufacture of uncreped, throughdried tissue sheets, improved efficiencies and a higher quality end tissue product may be obtained by the addition of high temperature steam to the drying medium. In so doing, tissue sheets can be made which have improved absorbance and softness values. Further, the addition of high temperature steam to the drying medium allows the throughair drying process to be carried out more economically and under conditions which eliminate the scorching or burning of the drying web.
Hence, in one aspect, the invention resides in a method for making a throughdried tissue comprising depositing an aqueous suspension of papermaking fibers onto a forming fabric to form a wet web, transferring the wet web to a throughdrying fabric, and throughdrying the web to form a tissue sheet. The use of a drying medium having a high steam content of between 10 percent to 100 percent by volume of the medium allows the use of higher drying temperatures compared to a conventional heated air drying medium. The steam enhanced drying medium converts the free moisture within the fabric web to a water vapor and which is removed by the passage of the drying medium.
Hence, in another aspect, the invention resides in the foregoing method wherein the tissue sheet is dried using a drying medium in which high temperature steam is added to increase the temperature of the drying medium above the burning temperature of paper. The addition of live steam reduces the concentration of oxygen and allows a higher drying temperature to be achieved without scorch or burning of the paper web.
In a further aspect, the invention resides in supplying a drying medium to a fabric web in which the drying medium is substantially free of oxygen. As used herein, the term xe2x80x9csubstantially freexe2x80x9d is defined as having a free oxygen content of a sufficiently low concentration such that burning or scorching of a paper web is prevented when the drying medium temperature is above the traditional scorch or burning temperature for a heated air TAD process. Likewise, the term xe2x80x9creduced oxygen drying mediumxe2x80x9d is defined as a heated air medium in which a percentage of the drying medium comprises live steam. As such, the oxygen gas concentration within the drying medium is reduced compared to a heated drying medium without the addition of live steam. Typically, heated air will have an O2 percentage of about 21%.
The use of a reduced concentration oxygen gas or substantially oxygen-free drying medium allows a drying temperature higher than the scorch or burn temperature of a paper web to be used. The scorch temperature of a paper web may vary depending upon the thickness and quality of the referenced web. However, the scorch temperature for any particular paper web may be readily determined and such temperatures are, in fact, known values within the industry for various types of commercially produced webs.
The use of an elevated throughair drying temperature brings about an additional improvement in the water absorbency and softness of the tissue fabric by the provision of a supply-side drying temperature above the glass transition point of paper fiber. The elevated temperatures allow the paper fiber to mold and permanently set the pulp fibers in an altered and desired shape.
In yet a further aspect, the invention resides in the foregoing method wherein the introduction of pressurized steam into the drying medium increases the velocity of the drying medium. This, in turn, lowers the energy demand on electric blowers and fans proportional to the motive energy provided by the introduced steam.
In yet a further aspect, the invention resides in a papermaking process in which the drying medium, upon leaving the throughdried web, has a portion of the resulting exhaust stream discharged along annular gaps defined between a throughair dryer hood and the associated paper web and drum. The discharge of the used drying medium forms a curtain seal along the annular gap seals and dryer web entry slot, thereby preventing cooler, oxygen-rich ambient air from infiltrating into the drying medium loop. Simultaneously, the exhaust curtain seals allow the discharge of a portion of the used drying medium so as to maintain an equilibrium of the drying medium circulation loop.
In yet another aspect, the invention resides in a method of making a tissue sheet wherein the throughair drying step is carried out by a drying medium comprising substantially about 100 percent (by volume) live steam. The use of substantially about 100 percent live steam will greatly reduce and may eliminate the need for electric motors used to circulate the drying medium. As a result, increased efficiencies can be obtained by the cost savings reflected in the use of pressurized steam as opposed to electric blowers to move the drying medium.
These and other aspects of the invention will be described in greater detail in reference to the figures and specification set forth below.